1. Field of the Invention
The present invention relates to an apparatus for processing an image signal which is used to encode and/or decode a binary image signal.
2. Description of the Prior Art
According to a conventionally known apparatus for processing an image signal, an image such as a document, diagram, or the like is scanned and read by an image scanner to obtain an image signal. This image signal is binarized and further converted into a highly efficient compressed code data by way of a two-dimensional encoding system using, for example, the correlation between adjacent lines. This compressed code data is then transmitted or stored.
Such a conventional image signal processing apparatus needs a circuit for compression encoding the binary image signal and a circuit for decoding the compression encoded image signal into the binary image signal, respectively.
In the case where the two-dimensional encoding system is used, during encoding and decoding the reference line information, namely, the position information at the transition (i.e., change) positions where the value of the binary data in the preceding adjacent line changes from black to white or from white to black must be encoded and decoded. This is a great amount of information and, therefore, line memories, each having a capacity large enough so that all of the binary image data of at least one of the reference lines can be stored, are needed for the encoding circuit and decoding circuit, respectively. Thus, the circuit scale and cost extremely increase as the complexity and detail of the image increase.
There is also the problem that it takes a long processing time period for the step of detecting the position information of the transition positions from the binary image data written in the line memory.
Hitherto, in the encoding, the information indicative of the position of the transition pixel having a value different from the value of the pixel just before the transition pixel of the binary signal, namely, the address of the transition position, is detected by checking the pixels dot by dot to see if the pixel's value changes from white to black or from black to white or not.
However, to check the pixels dot by dot, if it is assumed that the time period of, e.g., 100 nsec is needed to check one dot, takes 0.8 second to check the transition position of the image data of one screen which consists of, e.g., one mega bytes. Consequently, if the processing time period for encoding after the detection of the transition is included, a few seconds are required to encode the image signal of one screen.
Furthermore, in the case of reproducing the image by decoding the binary image signal from the signal encoded as described above, the run length of the decoded line is calculated on the basis of the code word while using the information of the reference line. The binary image signal is generated on the basis of the run length calculated and the image is reproduced.
According to the conventional image signal processing apparatus, the circuit to generate the binary image signal based on the run length is constituted by a counter of, e.g., twelve bits. The run length is set to the 12-bit counter and counted down in response to a clock pulse which is supplied to a clock terminal. When the value of the counter becomes 0, a transition signal indicative of the change from black to white or from white to black is generated in the binary image signal.
In such a construction, similarly to the encoding circuit, when it is assumed that a duration of one clock pulse is, e.g., 100 nsec, it takes the time period of 0.8 seconds to generate the binary signal if the image data of one screen consists of, e.g., one mega bytes. If the time period necessary for the decoding process is included, a few seconds are required to reproduce the image of one screen.